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The discovery of superconductivity with a critical temperature of about 80 K in La 3 Ni 2 O 7 single crystals under pressure has received enormous attention. La 3 Ni 2 O 7 is not superconducting under ambient pressure but exhibits a transition at T   ∗  ≃ 115 K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La 3 Ni 2 O 7 features strong electronic correlations which significantly reduce the electron’s kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at T   ∗ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni- d 3 z 2 − r 2 orbital is removed due to the transition at T   ∗ . Our experimental results provide pivotal information for understanding the transition at T   ∗ and superconductivity in La 3 Ni 2 O 7 . The bilayer nickelate La 3 Ni 2 O 7 was recently shown to be superconducting at high-pressure. Here the authors reveal strong electronic correlations and the opening of a partial gap, providing key information for understanding the nature of the density-wavelike transition at ambient pressure and superconductivity in this compound.

Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Rud-dlesden-Popper nickelate La 3 Ni 2 O 7 , the atomic structure and electronic band structure of the trilayer nickelate La 4 Ni 3 O 10 under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P 2 1 / a space group to the tetragonal I 4/ mmm around 12.6–13.4 GPa is identified, accompanied by a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni 3 d z 2 orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La 4 Ni 3 O 10 . The trilayer nickelate La 4 Ni 3 O 10 shows some similarities with the bilayer La 3 Ni 2 O 7 and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates.

The latest discovery of high temperature superconductivity near 80 K in La 3 Ni 2 O 7 under high pressure has attracted much attention. Many proposals are put forth to understand the origin of superconductivity. The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking. Here we report our direct measurement of the electronic structures of La 3 Ni 2 O 7 by high-resolution angle-resolved photoemission spectroscopy. The Fermi surface and band structures of La 3 Ni 2 O 7 are observed and compared with the band structure calculations. Strong electron correlations are revealed which are orbital- and momentum-dependent. A flat band is formed from the Ni-3d z 2 orbitals around the zone corner which is ~ 50 meV below the Fermi level and exhibits the strongest electron correlation. In many theoretical proposals, this band is expected to play the dominant role in generating superconductivity in La 3 Ni 2 O 7 . Our observations provide key experimental information to understand the electronic structure and origin of high temperature superconductivity in La 3 Ni 2 O 7 . Recently, superconductivity near 80 K was observed in La3Ni2O7 under high pressure, but the mechanism is debated. Here the authors report angle-resolved photoemission spectroscopy measurements under ambient pressure, revealing flat bands with strong electronic correlations that could be linked to superconductivity.

Recent discoveries of superconductivity in Ruddlesden-Popper nickelates realize a rare category of superconductors. However, the use of high-pressure diamond anvil cells limits spectroscopic characterization of the density waves and superconducting gaps. Here, we systematically studied the pressure evolution of La 4 Ni 3 O 10 using ultrafast optical pump-probe spectroscopy. We found that the transition temperature and energy gap of density waves are suppressed with increasing pressure and disappear suddenly near 17 GPa where structural transition appears. In addition, the observation of a single density wave gap indicates that the spin density wave and charge density wave remain coupled as pressure increases, rather than decoupling. After the density wave collapse, a distinct low-temperature regime emerges, characterized by a small gap consistent with potential superconducting pairing. The separated phase region of superconductivity and density waves suggests that superconductivity in pressurized-La 4 Ni 3 O 10 competes strongly with density waves, offering new insights into the interplay between these two phenomena. The authors study the pressure evolution of the density waves and their interplay with superconductivity in La 4 Ni 3 O 10 using ultrafast optical pump-probe spectroscopy. At low pressure, they quantify the density-wave gap. After the density wave collapses above 17 GPa, they find a small gap, likely due to the superconductivity.

The recent discovery of superconductivity in La 3 Ni 2 O 7− δ under high pressure with a transition temperature around 80 K (ref. 1 ) has sparked extensive experimental 2 – 6 and theoretical efforts 7 – 12 . Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here we develop a new, energy-filtered, multislice electron ptychography technique, assisted by electron energy-loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity 13 , 14 . We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K-edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also reveal that stoichiometric La 3 Ni 2 O 7 has strong charge-transfer characteristics, with holes that are self-doped from Ni sites into O sites. The ligand holes mainly reside on the inner apical O and the planar O, whereas the density on the outer apical O is negligible. As the concentration of O vacancies increases, ligand holes on both sites are simultaneously annihilated. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed-matter physics. Direct visualization of oxygen vacancies and self-doped ligand holes reveals the role of ligand oxygen in La 3 Ni 2 O 7− δ and provides further understanding of superconducting nickelate materials.

Although high-transition-temperature (high- T c ) superconductivity in cuprates has been known for more than three decades, the underlying mechanism remains unknown 1 – 4 . Cuprates are the only unconventional superconductors that exhibit bulk superconductivity with T c above the liquid-nitrogen boiling temperature of 77 K. Here we observe that high-pressure resistance and mutual inductive magnetic susceptibility measurements showed signatures of superconductivity in single crystals of La 3 Ni 2 O 7 with maximum T c of 80 K at pressures between 14.0 GPa and 43.5 GPa. The superconducting phase under high pressure has an orthorhombic structure of Fmmm space group with the 3 d x 2 − y 2 and 3 d z 2 orbitals of Ni cations strongly mixing with oxygen 2 p orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the 3 d z 2 orbitals with the apical oxygen ions connecting the Ni–O bilayers. Thus, our discoveries provide not only important clues for the high- T c superconductivity in this Ruddlesden–Popper double-layered perovskite nickelates but also a previously unknown family of compounds to investigate the high- T c superconductivity mechanism. Signatures of superconductivity in single crystals of La 3 Ni 2 O 7 were observed at a maximum transition temperature of 80  K at pressures between 14.0  GPa and 43.5  GPa.

Understanding the interplay between superconductivity and magnetism has been a longstanding challenge in condensed matter physics. Here we report high pressure studies on the C -type antiferromagnetic semiconductor EuTe 2 up to 36.0 GPa. A structural transition from the I4/mcm to the C2/m space group is identified at ~16 GPa. Superconductivity is observed above ~5 GPa in both structures. In the low-pressure phase, magnetoresistance measurements reveal strong couplings between the local moments of Eu 2+ and the conduction electrons of Te 5 p orbits. The upper critical field of superconductivity is well above the Pauli limit. While EuTe 2 becomes nonmagnetic in the high-pressure phase and the upper critical field drops below the Pauli limit. Our results demonstrate that the high upper critical field of EuTe 2 in the low-pressure phase is due to the exchange field compensation effect of Eu 2+ and the superconductivity in both structures may arise in the framework of the Bardeen-Cooper-Schrieffer theory. Understanding the interplay between superconductivity and magnetism has been a longstanding challenge in condensed matter physics. Here, the authors uncover a sensitive coupling between the two within the pressure-tuned phase diagram of EuTe 2 and find that certain magnetic orders can stabilize conventional superconductivity far exceeding the Pauli limit.

Recent experimental observations have showed some signatures of superconductivity close to 80 K in La 3 Ni 2 O 7 under pressure and have raised the hope of achieving high-temperature superconductivity in bulk nickelates. However, a zero-resistance state—a key characteristic of a superconductor—was not observed. Here we show that the zero-resistance state does exist in single crystals of La 3 Ni 2 O 7− δ using a liquid pressure medium at up to 30 GPa. We also find that the system remains metallic under applied pressures, suggesting the absence of a metal–insulator transition proximate to the superconductivity. Moreover, analysis of the normal state T -linear resistance reveals a link between this strange-metal behaviour and superconductivity. The association between strange-metal behaviour and high-temperature superconductivity is very much in line with other classes of unconventional superconductors, including the cuprates and Fe-based superconductors. Further investigations exploring the interplay of strange-metal behaviour and superconductivity, as well as possible competing electronic or structural phases, are essential to understand the mechanism of superconductivity in this system. Some features resembling superconductivity at high temperature have been seen under pressure in La 3 Ni 2 O 7 , but a transition to a zero-resistance state has not been observed. Now transport studies demonstrate this transition, along with strange metallicity.

Charge and spin orders are intimately related to superconductivity in copper oxide superconductors. Elucidation of the competing orders in various nickel oxide compounds is crucial, given the fact that superconductivity has been discovered in Nd 0.8 Sr 0.2 NiO 2 films. Herein, we report structural, electronic transport, magnetic, and thermodynamic characterizations of single crystals of La 3 Ni 2 O 7 and La 3 Ni 2 O 6 . La 3 Ni 2 O 7 is metallic with mixed Ni 2+ and Ni 3+ valent states. Resistivity measurements yield two transition-like kinks at ∼ 110 and 153 K. The kink at 153 K is further revealed from magnetization and specific heat measurements, indicative of the formation of charge and spin density waves. La 3 Ni 2 O 6 single crystals obtained from the topochemical reduction of La 3 Ni 2 O 7 are insulating and show an anomaly at ∼176 K on magnetic susceptibility. The transition-like behaviors of La 3 Ni 2 O 7 and La 3 Ni 2 O 6 are analogous to those observed in La 4 Ni 3 O 10 and La 4 Ni 3 O 8 , suggesting that charge and spin density waves are a common feature in the ternary La−Ni−O system with mixed-valent states of nickel.